1. Field of the Invention
The present invention relates to a fuel cell system having a fuel cell that generates electric power by an electrochemical reaction between oxygen and hydrogen. The fuel cell system is suitably used for a movement body such as a vehicle, a ship and a portable generator.
2. Description of Related Art
In a fuel cell for generating electric power using a chemical reaction between hydrogen and air (oxygen), moisture (water) is generated in the fuel cell due to power generation. In a low temperature environment such as in winter, the water contained in the fuel cell is frozen. Therefore, operation of the fuel cell is not started, or output from the fuel cell is reduced. Accordingly, in a fuel cell for an electrical vehicle, a large heat source for heating all of the fuel cell is required to rapidly heat the fuel cell.
In JP-A-2000-294263, a fuel cell stack is divided into plural stack parts, and a first stack part of them is warmed up at first. Thereafter, a second stack part and the other stack parts are sequentially warmed up using a thermal loss and electric power due to the power generation from the first stack part. However, in this method, during the warm-up operation of the fuel cell, some stack parts generate electric power, and the other stack parts generate no electric power. The number of stack parts generating electric power is increased, and all stack parts generate electric power until the warm-up is ended. Therefore, power-switching devices such as breakers are needed for switching a power supply source, because power-generation stack parts and non-generation stack parts are mixed. A vehicle fuel cell generally generates large electric power, and its generated current frequently reaches some hundreds amperes (A). Since the power switching device must ensure some hundreds A, its size is required to be enlarged. Further, fuel gas such as oxygen and hydrogen is needed to be supplied to each stack part generating electric power, and it is unnecessary to supply the fuel gas to each stack part where no electric power is generated. Accordingly, plural valves for switching the fuel-gas supplying states are also required, and the size of the vehicle fuel cell is further enlarged.